Sensor-independent oscillation amplitude control

ABSTRACT

A device ( 100, 200 ) is described for generating an electric voltage by which a body of a capacitive and/or inductive sensor capable of vibration, such as a capacitive micromechanical rotational rate sensor in particular, is induced to vibrate.  
     In order to reduce the manufacturing cost of the sensor, a voltage generating device ( 109 ) is provided which induces a constant mechanical deflection of the body capable of vibration, this deflection being independent of the manufacturing tolerances of the sensor.

BACKGROUND INFORMATION

[0001] The present invention is directed to a device for generating anelectric voltage according to the preamble of the main claim.

[0002] A known rotational rate sensor produced by microsystem technologyhas an oscillating weight which oscillates about its axis of rotation.The oscillating weight has a comb structure, i.e., it is formed by acomb structure which alternately meshes with a first stationary combstructure and with a second stationary comb structure of the sensor asit oscillates. This arrangement forms two capacitors whose capacitanceschange in opposite directions over time. If the rotational rate sensorexperiences a rotational rate perpendicular to the axis of torsionalvibration of the oscillating weight, one side of the oscillating weightmoves toward the substrate of the rotational rate sensor and the otherside moves away from it. These changes in distance are measuredcapacitively by electrically conducting surfaces beneath the oscillatingweight. The comb structures which are stationary with respect to thesensor and the comb structure which is provided on the oscillatingweight are acted upon by an alternating voltage, thereby inducingoscillation of the oscillating weight.

[0003] To obtain a high signal-to-noise ratio of the test signal whichrepresents the rotational rate, the deflection of the moving structureof the sensor must be maximized.

[0004] In the case of a known capacitive micromechanical sensor, such asa rotational rate sensor manufactured by planar silicon processes inparticular, the change in capacitance depends not only on the deflectionof the moving structure but also on the gap distance. Gap distance isunderstood to refer to the average distance between the “teeth” of themovable comb structure and the two stationary comb structures in thecase of a stationary oscillating weight. Since the gap distance may varyfrom one sensor to the next due to the manufacturing technology, eachsensor must be adjusted individually to achieve maximum deflection, i.e,maximum vibration amplitude of the movable structure. Not only is thiscomplicated, but it may also result in the movable structure strikingagainst the stationary structure, which could damage the sensor.

ADVANTAGES OF THE INVENTION

[0005] The device according to the present invention having thecharacterizing features of the main claim has the advantage over therelated art in particular that, regardless of the manufacturingtolerances, it automatically adjusts a predefined deflection of theoscillating weight of a capacitive or inductive sensor. This eliminatesindividual manual adjustment of each sensor for setting a virtuallymaximum deflection of the oscillating weight in order to obtain amaximum signal-to-noise ratio. This makes it possible to manufacturecapacitive and inductive sensors such as rotational rate sensors inparticular less expensively.

DRAWINGS

[0006] The present invention is explained in greater detail below on thebasis of the example of a known capacitive rotational rate sensormanufactured by microsystem technology with reference to the drawing,where the same reference numbers denote the same or equivalent parts.

[0007]FIG. 1 shows the first part of a schematic diagram of a firstembodiment of a sensor-independent vibration amplitude regulating deviceaccording to the present invention;

[0008]FIG. 2 shows the second part of the schematic diagram of the firstembodiment of a sensor-independent vibration amplitude regulating deviceaccording to the present invention;

[0009]FIG. 3 shows the first part of the schematic diagram of the secondembodiment of a sensor-independent vibration amplitude regulating deviceaccording to the present invention;

[0010]FIG. 4 shows the second part of the schematic diagram of thesecond embodiment of a sensor-independent vibration amplitude regulatingdevice according to the present invention.

[0011] For the sake of simplicity, the schematic diagram of asensor-independent vibration amplitude regulating device according tothe present invention has been divided into FIGS. 1 and 2 plus 3 and 4.An output of a first part of the schematic diagram, labeled as A inFIGS. 1 and 3, is connected electrically to an input of a second part ofthe schematic diagram, labeled as E in FIGS. 2 and 4.

[0012] First part 100 of the schematic diagram of the first embodimentof the vibration amplitude regulating device according to the presentinvention, as illustrated in FIG. 1, shows at the left a schematicdiagram 101 of another comb structure arrangement having a combstructure movable with the oscillating weight and two stationary combstructures of the type described above. These additional comb structuresare used to sense the deflection of the oscillating weight. Diagram 101shows two capacitors 102 and 103, which are formed by the two combstructures, these comb structures being stationary with respect to thesensor and having the function of sensing the deflection, and by themovable comb structure oscillating between the two former combstructures.

[0013] Furthermore, first part 100 of the schematic diagram shows afirst signal path 107, a second signal path 108, an adder 110, ademodulator 111, an amplifier 121 and a common-mode regulating apparatus109.

[0014] First signal path 107 has a terminal 104, a C/U converter 112 andan amplifier 113. Terminal 104 is connected to the input of C/Uconverter 112, whose output is connected to the input of amplifier 113,and the output of amplifier 113 is connected to a first input of adder110. In an identical manner, second signal path 108 has a terminal 106,a C/U converter 114 and an amplifier 115. Terminal 106 is connected tothe input of C/U converter 114 whose output is connected to the input ofamplifier 115, and the input of amplifier 115 is connected to a secondinput of adder 110. The output of adder 110 is connected to a firstinput of demodulator 111 and its output is connected to third input ofamplifier 121.

[0015] C/U converters 112 and 114 are preferably optical amplifierswired as inverting amplifiers having on-chip capacitance C_(RK) in thefeedback; these are charge amplifiers.

[0016] Common-mode regulating apparatus 109 (CMRA) has an adder 120, aregulator 119, preferably an I regulator, a modulator 118, a capacitor116 having a capacitance C_(I) and a capacitor 117 also havingcapacitance C_(I). A first input of adder 120 is connected to the outputof C/U converter 112, i.e., the input of amplifier 113, and a secondinput of adder 120 is connected to the output of C/U converter 114,i.e., the input of amplifier 115. The only output of adder 120 isconnected to the input of regulator 119, and the output of regulator 119is connected to both the input of modulator 118 and to a regulatingterminal of amplifier 121. The output of modulator 118 is connected to afirst terminal of capacitor 116 and to a first terminal of capacitor117. The second terminal of capacitor 116 is connected to the input ofC/U converter 112, i.e., terminal 104, and the second terminal ofcapacitor 117 is connected to the input of C/U converter 114, i.e.,terminal 106.

[0017] The second part of the schematic diagram of the first embodimentof the vibration amplitude regulating device of a rotational ratesensor, as shown in FIG. 2, shows input E connected to output A shown inFIG. 1, a phase quadrature device 201, an output stage 203, a terminal204, a terminal 205, an adder 208, an amplifier 209, a rectifier 206 anda regulator 207, where regulator 207 forms part of an automatic gaincontrol (AGC).

[0018] Input E of the second part of the schematic diagram of thevibration amplitude regulating device of a rotational rate sensor shownin FIG. 2 is connected to the input of the phase quadrature device 201,the output of phase quadrature device 201 being connected to the inputof amplifier 202, the output of amplifier 202 being connected to aninput of output stage 203, and one output of output stage 203 beingconnected to terminal 204 and another output of output stage 203 beingconnected to terminal 205. The input of phase quadrature device 201 isalso connected electrically to the input of rectifier 206, whose outputis connected to the first input of adder 208, whose output is in turnconnected to the input of regulator 207, and finally, the output ofregulator 207 is connected to an additional input of output stage 203.The second input of adder 208 is connected to the output of amplifier209.

[0019] A setpoint voltage U_(setpoint) is applied to the input ofamplifier 209 and sets the desired maximum deflection of the oscillatingweight for all sensors of the same type.

[0020] The function of the vibration amplitude regulation of arotational rate sensor according to the present invention is describedin detail below. It is assumed that the oscillating weight oscillatesabout its resting position.

[0021] The time-dependent capacitance (C(t)) of capacitor 102 orcapacitor 103 for identical capacitors, i.e., comb structures, isdescribed in first approximation as: $\begin{matrix}\begin{matrix}{{C_{102}(t)} = {{n*} \in {*{\left( {\left( \left( {1_{0} + {{\delta 1}(t)}} \right) \right)*h} \right)/d}}}} \\{= {C_{0} + {\delta \quad {C(t)}}}}\end{matrix} & (1) \\\begin{matrix}{{C_{103}(t)} = {{n*} \in {*{\left( {\left( \left( {1_{0} + {{\delta 1}(t)}} \right) \right)*h} \right)/d}}}} \\{= {C_{0}—\quad \delta \quad {C(t)}}}\end{matrix} & (2)\end{matrix}$

[0022] where:

[0023] l₀: basic overlapping of the movable comb structure with thecorresponding stationary comb structure;

[0024] δl: deflection of the movable comb structure;

[0025] h: height of the movable comb structure;

[0026] d: gap distance of the movable comb structure from the stationarycomb structure, i.e., the distance (ideally always identical) betweenadjacent “teeth” or fingers of movable and stationary comb structures;

[0027] n: number of overlapping fingers of movable and stationary combstructures;

[0028] ε: dielectric constant of the medium, air in particular, betweenthe movable and the stationary comb structures;

[0029] δC: time-dependent change in capacitance as a function of thedeflection of the movable comb structure relative to the stationary combstructure;

[0030] C₀: resting capacitance, i.e., the capacitance of the capacitorformed by the movable comb structure and the stationary comb structurewhen the movable comb structure is stationary.

[0031] It holds that:

δC/C ₀ =δl/l ₀  (3)

[0032] i.e., the relative change in capacitance due to deflection of themovable comb structure is equal to δl/l₀. The movable comb structure isacted upon by an alternating voltage U_(HF) from a device (not shown) atfrequency f_(HF) via terminal 105. Frequency f_(HF) of alternatingvoltage U_(HF) is much higher than operating frequency f_(sensor)supplied to the sensor via the driving comb structures. For example,frequency f_(HF) of alternating voltage U_(HF) corresponds approximatelyto 16 times operating frequency f_(sensor), operating frequencyf_(sensor) amounting to approx. 1.5 kHz, for example. It is self-evidentthat this information applies only to examples of one specific sensor.

[0033] An alternating voltage having a frequency f_(HF) is applied toterminals 104 and 106, frequency f_(HF) being amplitude-modulated withthe operating frequency of sensor f_(sensor).

[0034] The time-dependent capacitance of first capacitor 102 isconverted by C/U converter 112 into a corresponding electric voltage,amplified by amplifier 113 and sent to adder 110. The capacitance ofsecond capacitor 103 showing an inverse time dependence in comparisonwith the capacitance of the first capacitor is converted by C/Uconverter 114 into a corresponding electric voltage, amplified byamplifier 115 and also sent to adder 110.

[0035] The alternating voltage delivered by adder 110 is sent todemodulator 111. Demodulator 111 demodulates, i.e., multiplies thealternating voltage delivered by adder 110 by the sign of alternatingvoltage U_(HF).

[0036] Adder 110 forms the difference between the electric signals infirst signal path 107 and second signal path 108, amplified by gainfactor g by amplifier 113 and amplifier 115; therefore, the alternatingvoltage delivered by demodulator 111 at its output is: $\begin{matrix}\begin{matrix}{U_{FE} = {2*g*\delta \quad {C/C_{RK}}*U_{HF}}} \\{= {2*g*{{\delta 1}/1_{0}}*C_{0/}C_{RK}*U_{HF}}}\end{matrix} & (4)\end{matrix}$

[0037] where:

[0038] g: gain factor;

[0039] C_(RK): feedback capacitance of C/U converter 112 and identicalC/U converter 114;

[0040] U_(HF): alternating voltage U_(HF);

[0041] U_(FE): the alternating voltage delivered by demodulator 111after demodulation, i.e., multiplication by sign U_(HF),

[0042] this means that, due to the differentiation of the electricsignals at the output of first signal path 107 and second signal path108 performed by adder 110, the common-mode component caused by restingcapacitance C₀ is eliminated.

[0043] An essential aspect of the present invention is providingmeasures so that U_(FE) is independent of the resting capacitance C₀ ofthe sensor, which is subject to certain fluctuations due tomanufacturing tolerances.

[0044] According to a preferred embodiment of the present invention,both electric voltage U_(LV1) between the output of C/U converter 112and amplifier 113 and electric voltage U_(LV2) between the output of C/Uconverter 114 and amplifier 115 are picked up, electric voltage U_(LV1)being sent to the first input of adder 120 and electric voltage U_(LV2)being sent to the second input of adder 120.

[0045] The electric voltage delivered by C/U converters 112 and 114 attheir outputs is:

U _(LV1,LV2)=(C ₀ +/−δC)/C _(RK) +U _(HF)  (5)

[0046] The result of addition of the electric voltages performed byadder 120 is an output voltage U_(add) of adder 120, for which it holdsthat: $\begin{matrix}\begin{matrix}{U_{add} = {f\left( {\left( {C_{0} + {\delta \quad C}} \right) + \left( {C_{0}—\quad \delta \quad C} \right)} \right)}} \\{= {f\left( C_{0} \right)}}\end{matrix} & (6)\end{matrix}$

[0047] i.e., the output voltage of adder 120 is a function of restingcapacitance C₀.

[0048] Output voltage U_(add) of adder 120 is sent to regulator 119,preferably an I regulator delivering an output voltage U_(I) which issent to an input of modulator 118 and also to the regulating terminal ofamplifier 121.

[0049] Modulator 118 also receives alternating voltage U_(HF), and theoutput signal delivered by modulator 118 goes to a first terminal ofeach capacitor 116 and 117, both having a capacitance C_(I). The secondterminal of capacitor 116 is connected to the input of C/U converter 112in signal path 107, and the second terminal of capacitor 117 isconnected to the input of C/U converter 114 in signal path 108.

[0050] Capacitors 116 and 117 receive a voltage via regulator 119 suchthat the output signal of adder 120 has an amplitude of approx. 0 volt,i.e., capacitors 116 and 117 almost completely compensate restingcapacitance C₀ of the respective sensors.

[0051] Common-mode regulating apparatus 109 (CMRA) therefore respondsonly to common-mode signals, i.e., direct voltage signals, at the inputend. The output of regulator 119 changes its voltage in regulatingoperation until there is no longer a common mode signal at the input ofadder 120. This condition is met when the following holds:

U _(HF) *C ₀ =−U _(I) *C _(I)  (7)

i.e., U _(I) =−C ₀ /C _(I) *U _(HF)  (8)

[0052] i.e., voltage U_(I) is directly proportional to restingcapacitance C₀.

[0053] Amplifier 121 performs an amplification g_(var) of voltage U_(FE)as a function of the particular resting capacitance via voltage U_(I)applied to amplifier 121, for which the following equation holds:

g _(var) =C _(I) /C ₀  (9)

[0054] For electric voltage U delivered at the output of amplifier 121,this yields:

U=2*g*δl/l ₀ *C _(I) /C _(RK) *U _(HF)  (10)

[0055] where δC/C₀=δ/l₀ (see equation (3)),

[0056] i.e., the voltage applied at the output of amplifier 121, i.e.,at output A, is independent of resting capacitance C₀ of the particularsensor whose vibrational amplitude is to be regulated. Voltage U andthus change 61 in the path of the movable sensor element depend only onlow-tolerance voltage U_(HF), which is determined by the electronicregulation and/or measurement devices, and basic overlap l₀. Basicoverlap l₀ is settable with a high precision, however, in particular inthe case of a micromechanical sensor manufactured from semiconductorlayers by using planar silicon processes.

[0057] Voltage U delivered by amplifier 121 is sent to phase quadraturedevice 201, which sends voltage U, 90° out of phase, to the input ofamplifier 202 and sends amplified out-of-phase voltage U to an input ofoutput stage 203.

[0058] Furthermore, voltage U delivered by amplifier 121 is sent to theinput of rectifier 206 via input E, i.e., the input of the phasequadrature device. Setpoint voltage U_(setpoint) amplified by amplifier209 is subtracted by adder 208 from voltage U rectified by rectifier206, and the output signal of adder 208 is sent to the input ofregulator 207. Regulator 207 changes the voltage at its output until itsinput voltage is virtually zero. Regulator 207, preferably a PIregulator and/or an automatic gain control regulator (AGC) controlsoutput stage 203 so that the output stage delivers a voltage to thedrive comb structures of the sensor (not shown) via terminals 204 and205, so that the vibrational amplitude of the oscillating sensorelement, i.e., the oscillating weight, is constant and virtually at amaximum.

[0059] The second embodiment of the vibration amplitude regulatingdevice according to the present invention as illustrated in FIGS. 3 and4 differs from the first embodiment illustrated in FIGS. 1 and 2 in thatinstead of setpoint voltage U_(setpoint), voltage U_(I) delivered at theoutput of regulator 119 is applied to the second input of adder 208;furthermore, voltage U_(I) is not applied to amplifier 121 in the secondembodiment, so the amplifier implements a constant gain g_(const). Thefollowing thus holds for the output voltage of amplifier 121:

U=2*g*δl/i ₀ *C ₀ /C _(RK) *U _(RF) *g _(const)  (11)

[0060] The regulator, i.e., AGC regulator 207 changes its output voltageuntil output voltage U of amplifier 121 corresponds to AGC referenceinput variable U_(I) (or a variable proportional thereto). As in thefirst embodiment, this also means that the amplitude of vibration of theoscillating sensor element, i.e., the oscillating weight, is independentof resting capacitance C₀, which is subject to manufacturing tolerances.

[0061] Gap distance manufacturing tolerances due to overetching now nolonger have any effect on the deflection and thus the speed of themovable sensor element. A more complex and thus more expensiveadjustment of each finished sensor to adjust the desired deflection isno longer necessary when using the sensor-independent vibrationalamplitude regulating device according to the present invention.

[0062] As explained above, the sensor-independent vibration amplituderegulating device according to the present invention regulates thevibration amplitude of the oscillating weight of a capacitive sensorsuch as a rotational rate sensor in particular. It is self-evident thatthe vibrational amplitude regulating device described here may also beused in a modified form to regulate the amplitude of vibration of theoscillating weight of an inductive sensor, e.g., such as a rotationalrate sensor in particular. Such a modified form of the vibrationamplitude regulating device according to the present invention takesinto account in particular the fact that instead of capacitances, thereare inductances which are subject to manufacturing tolerances, in aninductive sensor.

LIST OF REFERENCE NOTATION

[0063]100 first part of the schematic diagram of the vibration amplituderegulating device according to the present invention

[0064]101 schematic diagram of the comb structures of a capacitiverotational rate sensor for sensing the deflection of its oscillatingweight

[0065]102 capacitor

[0066]103 capacitor

[0067]104 terminal

[0068]105 terminal

[0069]106 terminal

[0070]107 first signal path

[0071]108 second signal path

[0072]109 common-mode regulating apparatus (CMRA)

[0073]110 adder

[0074]111 demodulator

[0075]112 C/U converter

[0076]113 amplifier

[0077]114 C/U converter

[0078]115 amplifier

[0079]116 capacitor

[0080]117 capacitor

[0081]118 modulator

[0082]119 regulator

[0083]120 adder

[0084]121 amplifier

[0085]200 second part of the schematic diagram of the vibrationamplitude regulating device according to the present invention

[0086]201 phase quadrature device

[0087]202 amplifier

[0088]203 output stage

[0089]204 terminal

[0090]205 terminal

[0091]206 rectifier

[0092]207 regulator

[0093]208 adder

[0094]209 amplifier

What is claimed is:
 1. A device (100; 200) for generating an electricvoltage whereby vibration of a body of a capacitive and/or inductivesensor capable of vibration, such as a capacitive micromechanicalrotational rate sensor in particular, is induced, characterized by avoltage generating device (109) which generates an electric voltage(U_(I)) which is proportional to the resting capacitance (C₀) and/or tothe induction of the magnetic field of the sensor.
 2. The device asrecited in claim 1, wherein the voltage generating device (109) formspart of a regulating circuit for regulating the amplitude of vibrationof the body.
 3. The device as recited in claim 1 or 2, wherein thevoltage generating device is a common-mode regulating apparatus (109)which responds only to common-mode signals at the input end.
 4. Thedevice as recited in claim 3, wherein the common-mode regulatingapparatus (109) has a first adder (120) and a regulator, preferably an Iregulator, (119), and/or the output of the regulator (119) changes itsvoltage in regulating operation until virtually no common-mode signal isapplied at the input of the regulator (109).
 5. The device as recited inone of claims 1 through 4, wherein the sensor (101) has two elements(102, 103) whose capacitance is variable over time in oppositedirections, the two elements being formed in part by the body capable ofvibration, and the change in capacitance of the elements (102, 103)being detected separately in a first and a second signal path (107,108).
 6. The device as recited in claim 5, wherein the output signal ofthe adder (120) is sent to the input of the regulator (119), and a firstinput of the adder (120) picks up a first signal in the first signalpath (107), and a second input of the adder (120) picks up a secondsignal in the second signal path (108).
 7. The device as recited inclaim 6, wherein the output signal of the regulator (119) is sent to theinput of a modulator (118) which modulates the output signal using thefrequency (f_(HF)) of the voltage (U_(HF)) supplied to the elements(102, 103) and/or the output signal of the modulator (118) is sent tothe first terminal of a first capacitor (116) having a first capacitance(C_(I)) and to the first terminal of a second capacitor (117) having asecond capacitance (C_(I)) and/or the second terminal of the firstcapacitor (116) is electrically connected to the first signal path(107), and the second terminal of the second capacitor (117) iselectrically connected to the second signal path (108).
 8. The device asrecited in claim 7, wherein the output signal of the regulator (119) issent to a first input of an amplifier (121) which amplifies a voltage(U_(FE)) applied to it by a factor (C_(I)/C₀) which depends on theparticular sensor and is proportional to or equal to the quotient of thefirst capacitance (C_(I)) and the resting capacitance (C₀).
 9. Thedevice as recited in one of the preceding claims, wherein the outputsignal of the first signal path (107) is sent to a first input of asecond adder (110) and the output signal of the second signal path (108)is sent to a second input of the second adder (110), the output signalof the second adder (110) being demodulated by a demodulator (111) andsent to the amplifier (121) for amplification.
 10. The device as recitedin claim 9, wherein the demodulator (111) performs a demodulation usingthe frequency (f_(HF)) of the voltage (U_(HF)) supplied to the elements(102, 103).
 11. The device as recited in one of claims 1 through 10,wherein the output voltage (U) of amplifier (121) is kept constant. 12.The device as recited in one of claims 2 through 7 or 9 through 10,wherein the output voltage (U) of the amplifier (121) is proportional tothe resting capacitance (C₀) of the sensor, and the electric voltage(U_(I)) is the reference variable of the regulating circuit forregulating the amplitude of vibration of the body capable of vibration.